The objective of this proposal is to develop and investigate sophisticated radiopharmaceuticals bearing positron radionuclides for the purpose of developing superior radiodiagnostic agents for imaging organs containing neoplastic diseases. The formation of cancer is normally accompanied by changes in metabolic features of neoplastic tissue when compared with normal tissue. Thus, it is our belief that the development of radiopharmaceuticals with specific metabolic processes will lead to determining the functional status and morphology of organs containing neoplastic diseases prior to, during and after treatment by monitoring externally physiological parameters of the tissue metabolism. The currently used analogues of natural metabolites suffers from several important limitations. For example, 75-selenomethionine is labeled with a radionuclide with a long physical half life (120 days) and a long biological half life. These factors result in patient exposure to near maximum acceptable levels of the radiation dose. Similarly amino acids labeled with a halogen such as 125I substituted for a methyl group do not retain enough biological activity of the natural metabolite to be useful. To obviate these problems we propose to synthesize natural occuring metabolites labeled with short lived radioisotopes 18F, 13N, and 11C. These couples with positron tomography will improve visualization, reduce the radiation dose, limit toxicity and enable repeated studies while retaining the unique biological activity of the "physiological" molecule.